Peculiar porous α-Fe2O3, γ-Fe2O3 and Fe3O4 nanospheres: Facile synthesis and electromagnetic properties

We reported a facile approach to prepare peculiar porous α-Fe2O3, γ-Fe2O3 and Fe3O4 nanospheres by combining a facile hydrothermal route with a calcination process in Ar or H2 atmosphere. The synthesized monodisperse porous α-Fe2O3 nanospheres with uniform average diameters of ~ 60 nm in fact contained randomly distributed pores. A close view further revealed that there are two types of pores, one is large mesopores (ca. 15–20 nm) in the center, and the other is small mesopores (ca. < 10 nm) in the outside. After calcining in Ar or H2, the obtained α-Fe2O3, γ-Fe2O3 and Fe3O4 nanospheres preserved the similar morphology and particle size as the uncalcined α-Fe2O3 nanospheres, indicating the as-prepared α-Fe2O3 nanospheres are stable under Ar and H2-annealing heat treatment. Comparing with all the paraffin composites, it was found that the porous α-Fe2O3 nanosphere/paraffin composites exhibit a higher permittivity level. A minimum reflection loss (RL) of − 25 dB was observed at ~ 13 GHz for the porous α-Fe2O3 nanosphere/paraffin composites with a thickness of 3.5 mm, and the effective absorption frequency (RL < − 10 dB) ranged from 9.9 to 15.1 GHz. The composites exhibited better absorption properties than the magnetic porous γ-Fe2O3 and Fe3O4 nanosphere/paraffin composites.